Aqueous or solvent based pressure sensitive adhesives are well-known and have found application in a variety of areas such as, for example, in the manufacture of tapes and labels. Good pressure sensitive adhesives have immediate adhesion to a surface which property is often referred to as "quick tack". They also require good resistance to failure when subjected to shearing forces after application. This property is referred to as "shear strength" or "shear resistance". They must also have excellent bond strength which is referred to as "peel". In the past commercial adhesives have generally comprised an acrylate ester or vinyl ester based polymer system with various amounts of other monomers, for example, polyfunctional adhesion promoting monomers, added to give desired properties.
Recent developments in the disposable diaper field have created a demand for pressure sensitive tapes, but many pressure sensitive adhesives are not suited for this use because the adhesives cannot tolerate the shear forces associated with that application. Although attempts have been made to enhance the shear strength of the polymer system, the quick tack and peel values of the resulting adhesion generally diminish with the increase in shear strength.
Another commercial area requiring exceptional pressure sensitive adhesives is the permanent label field. These labels must adhere well and retain their bond formation. High shear resistance is required since these labels quite often are exposed to high shear forces.
The following patents disclose a variety of pressure sensitive adhesives which are considered to be acrylic ester or vinyl ester based systems. Representative patents include:
U.S. Pat. No. 3,257,478 discloses pressure sensitive adhesives comprising vinyl acetate, octyl acrylate, ethyl acrylate and maleic anhydride. In addition, crosslinkable monomers such as N-methylolacrylamide are included.
U.S. Pat. No. 3,654,213 discloses a pressure sensitive adhesive comprising a polymer base of 2-ethylhexyl acrylate, vinyl acetate, acrylic acid and a mixed vinyl ester of an alkanoic acid containing 10-12 carbon atoms.
U.S. Pat. No. 3,697,618 discloses an acrylic pressure sensitive adhesive consisting of a vinyl ester of a saturated monocarboxylic acid, e.g. vinyl acetate, an alkyl acrylate, e.g. one having from 4-14 carbon atoms such as 2-ethylhexyl acrylate or dodecyl acrylate, and an alpha,beta-ethylenically unsaturated dicarboxylic acid with the vinyl acetate being present in a proportion of from 20-60 wt %, at least 35 wt % alkyl acrylate and from about 0.3 to 5 wt % acid.
U.S. Pat. No. 3,890,292 discloses a pressure sensitive adhesive comprising an alkyl acrylate, an alpha,beta-unsaturated carboxylic acid such as acrylic or methacrylic acid, a vinyl ester, e.g. vinyl acetate, and a plasticizer such as dibutyl phthalate. Emulsion polymerization techniques are employed.
U.S. Pat. No. 3,971,766 discloses a pressure sensitive adhesive containing a copolymer of an alkyl acrylate with 2-6 mole % acrylic or methacrylic acid and 0.002 to 0.05 mole % of a polyfunctional unsaturated monomer, e.g. diallyl phthalate or polyethylene glycol dimethacrylate for imparting internal cohesive strength.
U.S. Pat. No. 3,998,997 discloses pressure sensitive adhesives of interpolymers of acrylates and acrylic acid using a di-, tri-, or tetrafunctional vinyl crosslinking agent to enhance the cohesive strength of the polymer. The pressure sensitive adhesives are formed by emulsion polymerization with anionic, cationic, nonionic or amphoteric-type emulsifiers and surfactants suggested as being effective stabilizers for the polymerization medium.
U.S. Pat. No. 4,185,050 discloses a pressure sensitive adhesive comprising a terpolymer of an alkyl acrylate, a vinyl ester and an ethylenically unsaturated carboxylic acid. The pressure sensitive adhesive comprising a blend of a plurality of terpolymers formed from the above monomers with each terpolymer having a specific molecular weight range.
U.S. Pat. No. Re. 24,906 discloses an acrylic-based pressure sensitive adhesive formed by polymerizing a non-tertiary acrylate ester with a copolymerizable monomer having strongly polar groups such as acrylic or methacrylic acid. Typical esters employed are isoamyl acrylate, 2-ethylbutyl acrylate and 2-ethylhexyl acrylate. The polymers are produced by emulsion polymerization.
The inventor is also aware of a process to produce vinyl acetate-ethylene adhesive emulsions having a Tg of about -5.degree. to +25.degree. C. The polymerization of a recipe of vinyl acetate, ethylene, water, stabilizer and reducing agent is initiated at a temperature of 25.degree. C. and a pressure below the operating pressure. The heat of reaction is utilized to achieve the operating temperature and operating pressure.
U.S. Pat. No. 4,332,850 discloses a method for preparing vinyl acetate-ethylene copolymer emulsions particularly adapted for the preparation of nonwoven goods. The process for enhancing the water absorptivity of the copolymer comprises pressurizing the reactor with ethylene, initiating the polymerization reaction by adding catalyst at a temperature from about 10.degree.-35.degree. C. and bringing the reaction mixture to a reaction temperature of 45.degree.-85.degree. C. and operating pressure within 2 hours or less with the reaction temperature exceeding the initiation temperature by at least 20.degree. C.